Generating Test Signals
In-depth signal generator coverage coincided with the mid-1980s broadening of EE’s original reliability and evaluation engineering focus. Beginning in 1985, signal generators of various types were featured each March for seven years. An article about arbitrary function generators (Arbs) was published in November 1985.
Arbs initially attracted a great deal of attention but didn’t quickly displace traditional designs. The slow rate of adoption may have favored stories about conventional signal generators, function generators, and frequency synthesizers—rather than Arbs—that ran from 1986 through 1988.
Arbs reappeared in a September 1988 LeCroy article that featured one of the instruments generating a video test waveform. In 1991, Pragmatic Instruments contributed an article about choosing the right Arb. And, indeed, several brands of Arbs were available at that time. Today, Pragmatic Arbs are a Tegam product line.
Courtesy of GenRad/Teradyne
The 1990s saw greatly expanded communications coverage in EE. The 1990 story “Of Signal Purity, Switching Speed, and Complex Modulation” addressed key aspects of RF/microwave generators. Signal purity and complex modulation are closely related through a constellation diagram, and switching speed is fundamental to any hopping protocols. A similar theme was pursued in the 1991 article “Communications, EW, and Radar Demand More From Generators.”
Arbs were featured in Wavetek’s 1992 article “GUIs Simplify the Generation of Real-World Test Signals,” which is representative of the trend toward menu-driven instruments at that time. Wavetek later became Willtek, today part of Aeroflex.
Analogic, originally Gordon Engineering, founded by Bernard Gordon of high-speed ADC fame, contributed two articles that highlighted the shift to digital-based signal-generation techniques. In 1991, the company chose to concentrate solely on biomedical engineering, furthering work it already had done in CT and MRI development, and today is a leader in those areas.
Arbs, function generators, and VXI overlapped during the early to mid-1990s. The 1995 article “Why Special Signals Are Needed for Digital Communications Testing” highlighted the role Arbs were to fill in the future. A 1996 Pragmatic story described several typical applications, including automotive crash test simulation leading to airbag deployment. But some Arbs were getting a lot faster.
Digital communications are based on quadrature I/Q baseband modulation, and Arbs are ideal for its generation. This application greatly expanded the Arb market and was discussed in the 1994 Tektronix story “Universal Signal Sources for Real-World Simulations.” These instruments initially addressed disk drive testing, but the goal was direct generation of a complete RF signal, not just the modulation.
A 1997 Giga-tronics story described the constraints faced in the design of a high-performance VXI-based RF generator. Signal purity must be maintained although RF/microwave synthesizers quickly switch frequencies to simulate channel-hopping radios. Typically, the information content—the modulation—comes from somewhere else. The story “Generating Signals for Testing Wireless Communications Systems” explored this theme in the same February issue. Some generators had specific built-in protocol capabilities and others used Arbs, but virtually all RF/microwave signal generators had separate baseband modulation and high-frequency carrier sections.
The 2004 Tektronix article “Ruling the Waves” discussed communications test applications for the company’s direct RF Arbs. The 4-GS/s sampling rate provided sufficient oversampling to develop a good SNR for a 900-MHz IS95 downlink signal with 1.2288-Mchip/s modulation. A follow-up story, “DAC Interleaving in Ultra-High-Speed Arbs,” which ran in 2009, described the underlying technology of the latest 20-GS/s instruments.
Coverage of signal generators and related topics from 2000 to 2010 included articles on noise figure measurement, the special needs of audio equipment testing, and the various spectral weightings applied to noise. Today, Arb-like techniques are being used in solar cell simulators. Although the power levels can be very high, these instruments must maintain a specific current-voltage output relationship to mimic solar cell behavior.
As we have done during our first 50 years of publication, EE-Evaluation Engineering will continue to explore all aspects of signals—the inputs and outputs of any test. As a complement to this review of our signal generator coverage, signal analyzers will be highlighted in the September issue.